This invention relates to the field of navigation, and more particularly to a device and method which can be used in any type of moveable vehicle, be it aircraft, land vehicle or marine vessel, to provide a dependable and accurate true heading reference. That is, the present invention provides an accurate and constantly updated readout of the vehicles present direction of travel relative to the earth's latitude and longitude coordinate system.
The simplest type of heading reference is a magnetic compass, however this device does not yield true heading but magnetic heading and in order to convert this to true heading, the local magnetic variation must be known and this can be determined only if the vehicle's position on the earth is known. Also, magnetic compasses are subject to many errors, e.g., deviation caused by magnetic materials on board the vehicle, or even iron ore deposits in the earth below the vehicle. Also, magnetic compasses suffer from acceleration and turning errors. Further, since they sense and respond to the horizontal component of the earth's field, their sensitivity decreases at locations near the earth's magnetic poles where the horizontal component becomes small or nonexistent. Also, the earth's magnetic poles are constantly shifting. Further, after a magnetic compass has been perturbed by acceleration, it will continue to oscillate for a time after the acceleration ceases.
The present invention provides a true heading reference or compass in which the heading is calculated from vehicle velocity measurements obtained from a Global Positioning System (GPS) receiver and other velocity measurements obtained either from a self contained Doppler velocity measuring system, or from vehicle-mounted accelerometers, the outputs of which are integrated to obtain along-track and cross-track velocities. Thus an accurate true heading is obtained by calculation utilizing information obtained from a satellite navigation system, the GPS; and from self contained on board velocity measuring equipment. The results are therefore completely independent of the earth's magnetic field and the errors which can be caused thereby, and it provides accurate heading information anywhere on earth. An accurate heading reference is particularly important for vehicles operating in arctic and antarctic regions where the earth's field is not reliable.